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Name: Amanda M.
Status: student
Age: 20s
Location: N/A
Country: N/A
Date: 3/22/2004


Question:
Why does a helium balloon go forward in an accelerating car?


Replies:
Amanda,

When a car accelerates forward, everything in the car "tries" to keep from accelerating. This is called inertia. The more mass an object has, the more inertia it feels. Inertia causes you, and everything else, to feel pushed toward the back of the car. Even the air and the balloon feel pushed back. Helium is the lightest material, so it feels the least inertia. Since you, the air, and everything else in the car feel more inertia than the balloon, the balloon gets pushed out of the air's way by moving forward.

Dr. Ken Mellendorf
Physics Professor
Illinois Central College


Amanda,

Imagine what would happen if you completely filled the car with water except for a small bubble near the roof. Every time you accelerated the car forward the inertia of the water would result in an effective force towards the back of the car. In effect it would look the same as if the gravitational source was near the back of the car. The bubble, floating on "top" of the water (with respect to the gravitational source) would move to the front of the car.

The helium filled balloon is like the air bubble and the atmosphere around it is like the water. The balloon floats on the air. When the air experiences forces toward the back of the car the balloon floats to the front.

Greg Bradburn


Same reason a pendulum goes backward in an accelerating car. It's slightly mind-bending, but think about it.

Jim Swenson



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